Can't access dotnet's environment variable via build target

I'm trying to access the ASPNETCORE_ENVIRONMENT environment variable inside a Target's Build event.

The .csproj looks like this:

<Project Sdk="Microsoft.NET.Sdk.Web">

    <PropertyGroup>
        <TargetFramework>net6.0</TargetFramework>
    </PropertyGroup>

    <Target Name="MyPostBuild" BeforeTargets="Build">
        <Exec Command="echo Operating System: $(OS)"/>
        <Exec Command="echo ASPNETCORE_ENVIRONMENT: $(ASPNETCORE_ENVIRONMENT)"/>
    </Target>

</Project>

The launchSettings.json contain the environment variable:

"profiles": {
    "DemoProject": {
        "environmentVariables": {
            "ASPNETCORE_ENVIRONMENT": "Development"
        }
    }
} 

It outputs the $(OS) very well but it does not prints the ASPNETCORE_ENVIRONMENT:

Operating System Windows_NT

ASPNETCORE_ENVIRONMENT:

My question is how to access the ASPNETCORE_ENVIRONMENT value within a Build event in order to make the build event run only with a specific condition like this:

<Target Name="MyPostBuild" BeforeTargets="Build" Condition="'$(ASPNETCORE_ENVIRONMENT)' == 'Development'">
    Do something
</Target>